The present invention relates to a sheet discriminating apparatus for detecting the soiling of sheets such as bank notes and classifying the sheets in accordance with the soiling.
In general, when bank notes are soiled to an extent that they cannot be automatically processed by a mechanism, such soiled bank notes are recovered by a national bank or the like which issues bank notes. For this purpose, a bank note discriminating apparatus is installed in each bank branch to classify bank notes into fit notes which can be recirculated since the soiling is below an allowable limit and unfit notes which cannot be recirculated and should be withdrawn because the soiling exceeds the allowable limit. In a conventional note discriminating apparatus, each bank note is conveyed to measure an amount of light transmitted through or reflected by the note and is discriminated as a fit or unfit bank note.
When part of a bank note is subjected to measurement, detection precision varies greatly in accordance with the particular part of the bank note. For example, slight soiling of a white portion can be easily detected but the same soiling of a black portion cannot be detected. For this reason, in order to detect the soiling of bank notes, the soiling of a white portion is detected in accordance with an amount of light transmitted therethrough. However, in the conventional apparatus, the white portion cannot always be subjected to detection. For example when a hole is formed in a bank note, the amount of light transmitted through or reflected by a portion including the hole greatly varies, resulting in an erroneous detection.
Light from a light source irradiates the bank notes to detect the soiling of the bank notes. In this case, deterioration of the light source over time and variation in amount of light emitted therefrom greatly influence judging results.
A threshold value for fit and unfit notes should be determined in accordance with the necessary number of bank notes for circulation and the number of bank notes in actual circulation. The greater the latter number becomes, the more strict the threshold criterion, and the smaller the latter number, the less strict the threshold criterion. In the prior art apparatus, the threshold value is fixed, which does not meet the requirements for actual discrimination.
These drawbacks result in an erroneous detection of the soiling of not only bank notes but also other sheets.